Drones are now widely used for material transportation because of their ability to take off and land vertically [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}. Aerial transportation, which is not limited to terrain, plays an increasingly important role in emergency cases [11]}, [12]}. Therefore, relevant research have always been concerned. When exploring drone delivery, researchers mainly adopt two methods, one of which is to use clamping claws or mechanical arms for transportation [13]}, [14]}, [15]}, [16]}. In this mode, the attitude of the load is highly controllable, which makes it convenient for obstacles avoidance. Another way of transportation is to suspend the load by sling ropes [17]}, [18]}, which ensures the flexibility of the drone itself and reduces additional cost on claws and arms. However, the load suspended by the cable inevitably sways under the drones, which may bring hidden danger to system security. For some particular transportation tasks, such as medical supplies, the damage caused by the sway of the load is unacceptable. Besides, a single drone usually does not have the ability to lift a heavy or a large-sized load. To solve these problems, it is a very natural idea to utilize two or even more drones to complete the tasks by cooperative control. Noting that effective control of the drone itself is already a very challenging work because the drone is underactuated with fewer control inputs than its degrees of freedom (DOFs), cooperative control for the double-drone transportation system is much more difficult, and it is still a fairly open problem so far.
